The present invention relates to image preprocessing, and more specifically to an image thinning process.
Typically, after a character is scanned and the features to be analyzed are detected, the detected image (characters for an OCR machine) is compared with a mask stored in memory for recognition. If there is sufficient correspondence between the detected image and the mask, the character is recognized. However, it has long been recognized as desirable to find the skeleton of an image during image preprocessing to facilitate computer interpretation of images in optical character recognition and robotic systems. Present thinning techniques first require detection and isolation of the entire character prior to thinning. See, for example, U.S. Pat. No. 3,846,754, issued to Oka et al. on Nov. 5, 1974; U.S. Pat. No. 3,940,737, issued to Beun on Feb. 24, 1976; U.S. Pat. No. 3,975,709, issued to Beun et al. on Aug. 17, 1976; U.S. Pat. No. 4,034,344, issued to Savaga et al. on July 5, 1977; U.S. Pat. No. 4,115,760 issued to Ito on Sept. 19, 1978; and U.S. Pat. No. 4,162,482, issued to Su on July 24, 1979. Thereafter, complicated computations are performed on the detected image including finding edge gradients, chordal distances, etc. to find the skeleton of the image. With such techniques, one of the advantages of thinning, i.e., helping with line finding and the segmentation of lines into characters, is not used.
U.S. Pat. No. 3,339,179 issued to Shelton, Jr., et al. on Aug. 29, 1967 discloses an image thinning technique based on eliminating all the pixels not necessary for connectivity. Specifically, a pattern is obtained by iteratively operating on the pattern to be identified with an incidence matrix. The result of operating on the input pattern by the incidence matrix is to generate an output pattern having a ridge of relatively high values along the center and lower values on either side of the ridge. Intersection points and end points are also determinable by the relative amplitudes of the values along the ridges. Such a thinning technique is not deterministic or universally applicable, but requires adjustments in the preservation values for different characters, e.g., by adjusting a potentiometer.